1. Field of the Invention
This invention relates to bearing blocks of the type in which a bearing supporting bore is formed with half of the bore in a support structure and the other half in a bearing cap which is secured to the support structure, in which the cap must be precisely refitted to the support structure after machining operations on the bore are carried out. In particular, this invention relates to a sintered powder metal bearing cap having bosses for providing precise relocation of the cap relative to the support structure.
2. Discussion of the Prior Art
The essential function of a bearing cap is to retain and locate a rotary shaft, or a bearing for a rotary shaft which in turn retains and locates the shaft, relative to a support structure. For example, the main bearing cap of an engine bolts to a bulkhead of the engine crankcase and together with the bulkhead retains and locates the crankshaft journal in place while the crankshaft is rotating. The crankshaft journal runs against two half shell bearings which are fitted to the main bearing cap and the engine bulkhead semi-circular bores, respectively.
In this case, for vibration free, low friction and quiet running, the roundness of the bore produced by the main bearing cap and the bulkhead is very important. This roundness is achieved by a machining operation called line boring. The main bearing caps are bolted to the bulkheads of the engine block, and then a boring bar fitted with a cutting tool is used to machine the bores in the assembly. This ensures the two half rounds formed by the main bearing cap and the bearing block form as near to a perfect circle as possible. A finishing operation involving a grinding hone is often used to achieve the extremely fine tolerances needed for quiet running and efficient engine performance.
However, to install the crankshaft, it is necessary to remove the main bearing caps from the engine block. After the crankshaft is put in place, it is necessary to reposition the main bearing caps to the bulkhead so that they are replaced in the identical position they occupied during the line boring operation. Any deviation from that original position produces an out-of-round condition that, in turn, leads to vibration, noise and possibly stiff, high friction crankshaft operation.
There are a number of conventional structures for re-locating and attaching the main bearing caps to bulkheads when installing the crankshaft. One such structure is shown in FIG. 1. In this instance, the main bearing cap C has a very precisely machined, snap-width W, which is the distance across the long axis of the main bearing cap across the foot sections T of the bearing cap. Similarly, a precision channel P is machined in the engine block bulkhead B to produce a controlled interference fit with the feet T when the main bearing cap C is refitted after crankshaft installation.
This method does not, however, provide relocation in the fore and aft direction (i.e., in the direction of the axis of the journal bore J). The bolt holes H themselves are used to control the axial repositioning, and since there is a substantial clearance between the bolts F and the bolt holes H of the main bearing cap C, this relocation accuracy is generally poor.
In addition, the interference fit between the main bearing caps C and the channel P in the engine block B in this structure is a variable which affects the final roundness of the bore J after re-installation. A highly stressed main bearing cap C may stress relieve during engine operation, thereby changing the roundness of the bore. Also, the precision machining operations required on the main bearing caps C to define the snap width W and on the block B to form the channel P, so as to avoid an overstressed or loose main bearing cap in this structure, are relatively expensive.
Another known method of location and attachment is shown in FIG. 2. This involves the use of hollow dowels D. These dowels D are pressed into counter-bored holes L in the engine block bulkhead P. The dowels D then locate in precisely machined counterbores M in the corresponding main bearing cap foot sections T. The accuracy of installation of the hollow dowels D is dependent upon the precision counterboring of the engine block and the main bearing cap. Both of these operations have a finite tolerance which, when stacked up with the tolerance on the dowel D outer diameter, can produce an unacceptable variation in location of the main bearing cap C. Additionally, this procedure has the added expense of purchasing precision hollow dowels, their handling and installation, and the costly machining of precision bores L in the bulkhead B and M in the main bearing caps C.
In many cases where hollow dowels as shown in FIG. 2 are used, the engine block channel/main bearing cap snap width relocation method of FIG. 1 is also used. This combination is expensive and, in fact, can produce a situation where the interference fits between the snap-width and channel are in conflict with the interference fits between the hollow dowels and the main bearing cap or bulkhead holes.